Field emitters are useful in various applications such as flat panel displays and vacuum microelectronics. Field emission based displays in particular have substantial advantages over other available flat panel displays, including lower power consumption, higher intensity, and generally lower cost. Currently available field emission based flat panel displays however disadvantageously rely on micro-fabricated metal tips which are difficult to fabricate. The complexity of the metal tip fabrication processes, and the resulting low yield, lead to increased costs which disadvantageously impact on overall display system costs.
Field emission is a phenomenon which occurs when an electric field proximate the surface of an emission material narrows a width of a potential barrier existing at the surface of the emission material. This narrowing of the potential barrier allows a quantum tunnelling effect to occur, whereby electrons cross through the potential barrier and are emitted from the material. The quantum mechanical phenomenon of field emission is distinguished from the classical phenomenon of thermionic emission in which thermal energy within an emission material is sufficient to eject electrons from the material.
The field strength required to initiate field emission of electrons from the surface of a particular material depends upon that material's effective "work function." Many materials have a positive work function and thus require a relatively intense electric field to bring about field emission. Other materials such as cesium, tantalum nitride and trichromium monosilicide, can have low work functions, and do not require intense fields for emission to occur. An extreme case of such a material is one with negative electron affinity, whereby the effective work function is very close to zero (&lt;0.8 eV). It is this second group of materials which may be deposited as a thin film onto a conductor, to form a cathode with a relatively low threshold voltage to induce electron emissions.
In prior art devices, the field emission of electrons was enhanced by providing a cathode geometry which increases local electric field at a single, relatively sharp point at the tip of a cone (e.g., a micro-tip cathode). For example, U.S. Pat. No. 4,857,799, which issued on Aug. 15, 1989, to Spindt et al., is directed to a matrix-addressed flat panel display using field emission cathodes. The cathodes are incorporated into the display backing structure, and energize corresponding cathodoluminescent areas on an opposing face plate. Spindt et al. employ a plurality of micro-tip field emission cathodes in a matrix arrangement, the tips of the cathodes aligned with apertures in an extraction grid over the cathodes. With the addition of an anode over the extraction grid, the display described in Spindt et al. is a triode (three terminal) display.
Micro-tip cathodes are difficult to manufacture since the micro-tips have fine geometries. Unless the micro-tips have a consistent geometry throughout the display, variations in emission from tip to tip will occur, resulting in uneven illumination of the display. Furthermore, since manufacturing tolerances are relatively tight, such micro-tip displays are expensive to make. Thus, to this point in time, substantial efforts have been made in an attempt to design cathodes which can be mass produced with consistent close tolerances.
In addition to the efforts to solve the problems associated with manufacturing tolerances, efforts have been made to select and use emission materials with relatively low effective work functions in order to minimize extraction field strength. One such effort is documented in U.S. Pat. No. 3,947,716, which issued on Mar. 30, 1976, to Fraser, Jr. et al., directed to a field emission tip on which a metal adsorbent has been selectively deposited. Further, the coated tip is selectively faceted with the emitting planar surface having a reduced work function and the non-emitting planar surface as having an increased work function. While micro-tips fabricated in this manner have improved emission characteristics, they are expensive to manufacture due to the required fine geometries. The need for fine geometries also makes emission consistency between micro-tips difficult to maintain. Such disadvantages become intolerable when large arrays of micro-tips, such as in flat display applications, are required.
Additional efforts have been directed to finding suitable geometries for cathodes employing negative electron affinity substances as a coating for the cathode. For instance, U.S. Pat. No. 3,970,887, which issued on Jul. 20, 1976, to Smith et al., is directed to a microminiature field emission electron source and method of manufacturing the same. In this case, a plurality of single crystal semiconductor raised field emitter tips are formed at desired field emission cathode sites, integral with a single crystal semiconductor substrate. The field emission source according to Smith et al. requires the sharply tipped cathodes found in Fraser, Jr. et al. and is therefore also subject to the disadvantages discussed above.
U.S. Pat. No. 4,307,507, issued Dec. 29, 1981 to Gray et al. and U.S. Pat. No. 4,685,996 to Busta et al. describe methods of fabricating field emitter structures. Gray et al. in particular is directed to a method of manufacturing a field-emitter array cathode structure in which a substrate of single crystal material is selectively masked such that the unmasked areas define islands on the underlying substrate. The single crystal material under the unmasked areas is orientation-dependent etched to form an array of holes whose sides intersect at a crystallographically sharp point. Busta et al. is also directed to a method of making a field emitter which includes anisotropically etching a single crystal silicon substrate to form at least one funnel-shaped protrusion on the substrate. Busta et al. further provides for the fabrication of a sharp-tipped cathode.
Sharp-tipped cathodes are further described in U.S. Pat. No. 4,885,636, which issued on Aug. 8, 1989, to Busta et al. and U.S. Pat. No. 4,964,946, which issued on Oct. 23, 1990, to Gray et al. Gray et al. in particular discloses a process for fabricating soft-aligned field emitter arrays using a soft-leveling planarization technique, (e.g., a spin-on process).
While the use of low effective work-function materials improves emission, the sharp tipped cathodes referenced above are still subject to the disadvantages inherent with the required fine geometries: sharp-tipped cathodes are expensive to manufacture and are difficult to fabricate such that consistent emission is achieved across an array. Flat cathodes help minimize these disadvantages. Flat cathodes are much less expensive and less difficult to produce in large numbers (such as in an array) because the microtip geometry is eliminated. In Ser. No. 07/851,701, which was filed on Mar. 16, 1992, and entitled "Flat Panel Display Based on Diamond Thin Films," an alternative cathode structure was first disclosed. Ser. No. 07/851,701 discloses a cathode having a relatively flat emission surface as opposed to the aforementioned micro-tip configuration. The cathode, in its preferred embodiment, employs a field emission material having a relatively low effective work function. The material is deposited over a conductive layer and forms a plurality of emission sites, each of which can field-emit electrons in the presence of a relatively low intensity electric field.
A relatively recent development in the field of materials science has been the discovery of amorphic diamond. The structure and characteristics of amorphic diamond are discussed at length in "Thin-Film Diamond," published in the Texas Journal of Science, vol. 41, no. 4, 1989, by C. Collins et al. Collins et al. describe a method of producing amorphic diamond film by a laser deposition technique. As described therein, amorphic diamond comprises a plurality of micro-crystallites, each of which has a particular structure dependent upon the method of preparation of the film. The manner in which these micro-crystallites are formed and their particular properties are not entirely understood.
Diamond has a negative election affinity. That is, only a relatively low electric field is required to narrow the potential barrier present at the surface of diamond. Thus, diamond is a very desirable material for use in conjunction with field emission cathodes. For example, in "Enhanced Cold-Cathode Emission Using Composite Resin-Carbon Coatings," published by S. Bajic and R. V. Latham from the Department of Electronic Engineering and Applied Physics, Aston University, Aston Triangle, Burmingham B4 7ET, United Kingdom, received May 29, 1987, a new type of composite resin-carbon field-emitting cathode is described which is found to switch on at applied fields as low as approximately 1.5 MV m.sup.-1, and subsequently has a reversible I-V characteristic with stable emission currents of greater than or equal to 1 mA at moderate applied fields of typically greater than or equal to 8 MV m.sup.-1. A direct electron emission imaging technique has shown that the total externally recorded current stems from a high density of individual emission sites randomly distributed over the cathode surface. The observed characteristics have been qualitatively explained by a new hot-electron emission mechanism involving a two-stage switch-on process associated with a metal-insulator-metal-insulator-vacuum (MIMIV) emitting regime. However, the mixing of the graphite powder into a resin compound results in larger grains, which results in fewer emission sites since the number of particles per unit area is small. It is preferred that a larger amount of sites be produced to produce a more uniform brightness from a low voltage source.
Similarly, in "Cold Field Emission From CVD Diamond Films Observed In Emission Electron Microscopy," published by C. Wang, A. Garcia, D. C. Ingram, M. Lake and M. E. Kordesch from the Department of Physics and Astronomy and the Condensed Matter and Surface Science Program at Ohio University, Athens, Ohio on Jun. 10, 1991, there is described thick chemical vapor deposited "CVD" polycrystalline diamond films having been observed to emit electrons with an intensity sufficient to form an image in the accelerating field of an emission microscope without external excitation. The individual crystallites are of the order of 1-10 microns. The CVD process requires 800.degree. C. for the depositing of the diamond film. Such a temperature would melt a glass substrate used in flat panel displays.
In sum the prior art has failed to: (1) take advantage of the unique properties of amorphic diamond; (2) provide for field emission cathodes having a more diffused area from which field emission can occur; and (3) provide for a high enough concentration of emission sites (i.e., smaller particles or crystallites) to produce a more uniform electron emission from each cathode site, yet require a low voltage source in order to produce the required field for the electron emissions.